The present invention relates to a pneumatic tire provided in a tread shoulder portion with a circumferential groove, more particularly to a heavy duty tire improved in uneven wear in the vicinity of the circumferential groove.
In the case of heavy duty tires such as truck/bus tires having a rib-type or block-type tread pattern comprising a circumferentially extending wide groove in each tread shoulder portion, as well known in the art, so called shoulder wear, railway wear and punching, namely, uneven tread wear in the vicinity of the axially outermost circumferential groove are very liable to occur.
Heretofore, a main cause of such uneven tread wear is believed to be an uneven ground pressure distribution due to the difference in the outer diameter of the tire existing between the tread central region and shoulder region, and/or a sufficient rigidity of tread elements such as rib and block existing in the vicinity of the axially outermost circumferential groove.
However, as a result of extensive studies, the present inventor found that the radial run-out (RRO) of a tread reinforcing belt layer is largely involved in such uneven wear, and that the uneven wear can be improved if the radial run-out of the belt layer is decreased.
Usually, pneumatic tires inclusive of heavy duty tires are vulcanized in a segmented mold M, whose surface U for molding the tread portion of the tire is, as shown in FIG. 6, made up of a plurality of circumferentially arranged segments Sg. The inventor found that the tread reinforcing belt layers of the tires vulcanized in such segmented mold M tend to have a radial run-out increased at a particular order which is the same as the number (n) of the sprit positions (j) of the segments Sg counted around the circumference of the tire, and realized that the radial run-out of the tread reinforcing belt can be reduced by reducing the n-th order component thereof (of course, other components should be small enough at the same time). It is supposed that at the circumferential positions where the radial run-out of the belt layer is larger, the ground pressure becomes higher, and as a result, local initial wear starts and such wear develops into the above-mentioned uneven tread wear concerned. However, if the RRO of the belt layer is reduced, local initial wear is inhibited and accordingly, the uneven tread wear can be improved.